i2S microphone I need help

[accacca]

I managed to do the first steps with ADF
Starting from the esp-rtsp example I installed an rtsp server and I can do video stream on VLC.
Now I wanted to add audio stream and I thought it would be easier....
My board has an i2S microphone, at the moment the audio_board_init function defined in my custom board.c file returns NULL so the audio stream is ignored.
I don't understand how should I configure audio_board_init to handle I2S microphone.
Is there any example available
thanks